Lithium ion conductive glass-ceramics

ABSTRACT

There are provided glass-ceramics having a high lithium ion conductivity which include in mol % 
     
         ______________________________________                                    
 
    
     P 2  O 5      38-40%                                                
TiO 2            25-45%                                                
M 2  O 3  (where M is Al or Ga)                                     
                     5-15%                                                
Li 2  O          10-20%                                                
______________________________________                                    
 
     and contain Li 1+X  (Al, Ga) X  Ti 2-X  (PO 4 ) 3  (where X is 0 to 0.8) as a main crystal phases. There are also provided glass-ceramics having a high lithium ion conductivity which include in mol %______________________________________P 2  O 5     26-40%SiO 2           0.5-12%TiO 2           30-45%M 2  O 3  (where M is Al or Ga)                    5-10%Li 2  O         10-18%______________________________________ 
     and contain Li 1+X+Y  M X  Ti 2-X  Si Y  P 3-Y  O 12  (where 0&lt;X≦0.4 and 0&lt;Y≦0.6) as a main crystal phase.

BACKGROUND OF THE INVENTION

This invention relates to a lithium ion conductive glass-ceramicssuitable for use as all solid electric cell, sensors and electrochemicaldevices of various types.

Recent development in electronics has brought about high-performanceelectronic devices of a compact and light-weight design and, as a powersource of such electronic devices, development of an electric cell of ahigh energy density and a long life is strongly desired for.

Lithium has the highest oxidation-reduction potential of Li/Li⁺ of allmetal elements and has the smallest mass per 1 mol and, therefore,lithium cell can provide a higher energy density than other types ofcells. Moreover, if a lithium ion conductive solid electrolyte is used,this electrolyte can be made very thin and, therefore, a cell of a thinfilm can be formed and increase in energy density per unit volume canthereby be realized.

A lithium ion cell which has been realized to date uses an organicelectrolyte solution as its electrolyte and this makes it difficult toachieve a cell of a compact design such as a thin film design. Thislithium ion cell has additional disadvantages that it has likelihood ofleakage of electrolyte solution and likelihood of spontaneouscombustion. If this lithium ion cell is replaced by a cell employing aninorganic solid electrolyte, a wholly solid cell of a high reliabilitywill be realized. For this reason, studies and developments of a solidelectrolyte having a high conductivity have been vigorously made forrealizing a wholly solid lithium cell. Among known solid electrolyes,Li₃ N single crystal (Applied Physics letter, 30(1977) 621-22) andLiI-Li₂ S-P₂ S₅, LiI-Li₂ S-SiS₄ and LiI-Li₂ S-B₂ S₃ glasses (Mat. Res.Bull., 18(1983) 189) have high conductivity of 10⁻³ S/cm or over. Thesematerials, however, have the disadvantages that preparation and handlingof these materials are difficult and that decomposition voltage of thesematerials is so low that, when they are used for a cell, a sufficientlyhigh terminal voltage cannot be obtained.

An oxide lithium solid electrolyte does not have the above describeddisadvantages and has a decomposition voltage which is higher than 3Vand, therefore, it has possibility of usage as a wholly solid lithiumcell if it exhibits a high conductivity at room temperature. It is knownin the art that conductivity in an oxide glass can be increased byincreasing lithium ion concentration. However, there is limitation inincrease in the lithium ion concentration even if rapid quenching isemployed for glass formation and conductivity of this glass at roomtemperature is below 10⁻⁶ S/cm at the highest. An oxide ceramic havingthe highest conductivity at room temperature is Li_(1+X) Al_(X) Ti_(2-X)(PO₄)₃. When X is 0.3, the conductivity thereof is 7×10⁻⁴ S/cm at roomtemperature (J. Electrochem. Soc., 137 (1990) 1023). Oxide ceramics aresuperior in conductivity to glasses but have the disadvantages that theyrequire a troublesome process for manufacturing and that they aredifficult to form, particularly to a thin film.

In short, the prior art lithium ion solid electrolytes have the problemsthat they are either low in conductivity, hard to handle, hard to formto a compact design such as a thin film.

It is, therefore, an object of the invention to provide glass-ceramicswhich have solved these problems and exhibit a very high lithium ionconductivity in the order of 10⁻³ S/cm at room temperature.

SUMMARY OF THE INVENTION

As described above, Li_(1+X) Al_(X) Ti_(2-X) (PO₄)₃ ceramics exhibitconductivity of 10⁻⁴ S/cm or over at room temperature. These ceramics,however, have pores and a bad grain boundary which can not be eliminatedcompletely and existence of these pores and grain boundary results in adecrease in conductivity. If, therefore, glass-ceramics including theabove crystal are provided, there will be no pores and the grainboundary will be improved and, as a result, a solid electrolyte having ahigher conductivity is expected to be provided. Besides, glass-ceramicswhich share a feature of glass can be easily formed into various shapesincluding a thin film by utilizing this feature of glass. For thesereasons, glass-ceramics are considered to have practical advantages overceramics made by sintering.

As a result of studies and experiments made by the inventor of thepresent invention on the basis of the above described basic concept, theinventor has succeeded in obtaining glass-ceramics having a very highlithium ion conductivity in the order of 10⁻³ S/cm at room temperatureby producing glasses including the ingredients of the above describedcrystal and causing the crystal phase to grow from these glasses by heattreating these glasses.

A lithium ion conductive glass-ceramics achieving the above object ofthe invention comprise in mol %

    ______________________________________                                        P.sub.2 O.sub.5     38-40%                                                    TiO.sub.2           25-45%                                                    M.sub.2 O.sub.3 (where M is Al or Ga)                                                              5-15%                                                    Li.sub.2 O          10-20%                                                    ______________________________________                                    

and contains Li_(1+X) (Al, Ga)_(X) Ti_(2-X) (PO₄)₃ (where X is 0 to 0.8)as a main crystal phase.

In one aspect of the invention, said glass-ceramics comprise in mol %

    ______________________________________                                               P.sub.2 O.sub.5                                                                     38-40%                                                                  TiO.sub.2                                                                           30-45%                                                                  Al.sub.2 O.sub.3                                                                     5-15%                                                                  Li.sub.2 O                                                                           10-16%.                                                         ______________________________________                                    

In another aspect of the invention, said glass-ceramics comprise in mol%

    ______________________________________                                               P.sub.2 O.sub.5                                                                     38-40%                                                                  TiO.sub.2                                                                           25-45%                                                                  Ga.sub.2 O.sub.3                                                                     5-12%                                                                  Li.sub.2 O                                                                           10-20%.                                                                Li.sub.2 O                                                                          10-20%                                                           ______________________________________                                    

The inventor of the present invention has further succeeded in obtainingglass-ceramics having the high lithium ion conductivity in the order of10⁻³ S/cm at room temperature by producing glasses including ingredientsof P₂ O₅, SiO₂, TiO₂, M₂ O₃ (where M is Al or Ga) and Li2O and causing acrystal phase of a conductive crystal Li_(1-X+Y) M_(X) Ti_(2-X) Si_(Y)P_(3-Y) O₁₂ to grow from the glasses by hear treating these glasses.

Therefore, in another aspect of the invention, here is provided alithium ion conductive glass-ceramics comprising in mol %

    ______________________________________                                        P.sub.2 O.sub.5    26-40%                                                     SiO.sub.2          0.5-12%                                                    TiO.sub.2          30-45%                                                     M.sub.2 O.sub.3 (where M is Al or Ga)                                                             5-10%                                                     Li.sub.2 O         10-18%                                                     ______________________________________                                    

and containing Li_(1+X+Y) M_(X) Ti_(2-X) Si_(Y) P_(3-Y) O₁₂ (where0<X≦0.4 and 0<Y≦0.6) as a main crystal phase.

In another aspect of the invention, said glass-ceramics comprise in mol%

    ______________________________________                                               P.sub.2 O.sub.5                                                                     26-40%                                                                  SiO.sub.2                                                                           0.5-12%                                                                 TiO.sub.2                                                                           32-45%                                                                  Al.sub.2 O.sub.3                                                                     5-10%                                                                  Li.sub.2 O                                                                           10-18%.                                                         ______________________________________                                    

In another aspect of the invention, said glass-ceramics comprise in mol%

    ______________________________________                                               P.sub.2 O.sub.5                                                                     26-40%                                                                  SiO.sub.2                                                                           0.5-12%                                                                 TiO.sub.2                                                                           32-45%                                                                  Ga.sub.2 O.sub.3                                                                     5-10%                                                                  Li.sub.2 O                                                                           10-18%.                                                         ______________________________________                                    

According to the invention, there are provided lithium ion conductiveglass-ceramics which exhibit a very high conductivity in the order of10⁻³ S/cm at room temperature. In addition to having the highconductivity, the glass-ceramics made according to the invention havesuch an excellent formability that they can be easily formed intovarious shapes including a thin film, and they are thermally andchemically stable so that they are suitable for use as electrolytes ofwholly solid cells, sensors and other various electrochemical devices.

DETAILED DESCRIPTION OF THE INVENTION

The compositions of the glass-ceramics made according to the inventionare expressed on the basis of compositions of oxides as in their baseglasses. The above described content ranges of the respectiveingredients have been selected for the reasons stated below.

In the ternary system P₂ O₅ -TiO₂ -Li₂ O, glass forming region exists ina very narrow range and the composition identical with that of Li_(1+X)Al_(X) Ti_(2-X) (PO₄)₃ does not form glass when X is 0 (Bulletin of theChemical Society of Japan, 51(1978) 2559). In the P₂ O₅ -TiO₂ -(Al₂ O₃,Ga₂ O₃)-Li₂ O system including Al₂ O₃ or Ga₂ O₃, a glass forming rangehas not been reported yet. Neither has been reported any glass-ceramicwhich has been prepared from such systems for obtaining a high lithiumion conductivity.

The inventor of the present invention has examined the glass formingrange of the P₂ O₅ -TiO₂ -(Al₂ O₃, Ga₂ O₃)-Li₂ O system by employing aconventional glass melting method and obtained lithium ion solidelectrolytes of a high conductivity which can be glassified within thefollowing composition ranges (expressed in mol %) and can grow, as aresult of heat treatment, Li_(1+X) (Al, Ga)_(X) Ti_(2-X) (PO₄)₃ as amain crystal phase.

In the case of the system including Al₂ O₃,

    ______________________________________                                               P.sub.2 O.sub.5                                                                     38-40%                                                                  TiO.sub.2                                                                           30-45%                                                                  Al.sub.2 O.sub.3                                                                     5-15%                                                                  Li.sub.2 O                                                                           10-16%.                                                         ______________________________________                                    

In the case of a system including Ga₂ O₃,

    ______________________________________                                                P.sub.2 O.sub.5                                                                     38-40%                                                                  TiO.sub.2                                                                           25-45%                                                                  Ga.sub.2 O.sub.3                                                                     5-12%                                                                  Li.sub.2 O                                                                          10-20%                                                          ______________________________________                                    

In the case of a system including both Al₂ O₃ and Ga₂ O₃,

    ______________________________________                                               P.sub.2 O.sub.5                                                                        38-40%                                                               TiO.sub.2                                                                              25-45%                                                               Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3                                                      5-15%                                                               Li.sub.2 O                                                                             10-20%                                                        ______________________________________                                    

It has been found that a glass forming region exists beyond the abovedescribed composition ranges but, after a heat treatment, an electrolytehaving a high conductivity could not be obtained from such compositionrange outside of the above described composition ranges. The abovedescribed composition ranges of the glass-ceramics of the invention havebeen determined on the basis of these experiments.

In this system, a part of Al or Ga ingredient may be replaced by one ofsuch trivalent metal elements as B, In, Sc, Fe and Cr. In this case,however, the amount of Al or Ga replaced by such metal element shouldnot exceed 5%. If the amount of the replaced metal element exceeds 5%,conductivity will drop significantly.

A method for manufacturing the conductive glass-ceramics of the P₂ O₅-TiO₂ -(Al₂ O₃, Ga₂ O₃)-Li₂ O system will now be described.

Starting materials are weighed at a predetermined ratio and mixeduniformly and the mixed materials are thereafter put in a platinumcrucible and heated and melted in an electric furnace. First, gascomponents coming from the raw materials are evaporated at 700° C. andthen the temperature is raised to 1400° C. to 1450° C. and the materialsare melted at this temperature for about one to two hours. Then the meltis cast onto a stainless steel plate to form a sheet glass. Theresultant glass is subjected to heat treatment within the temperaturerange from 800° C. to 1000° C. for 10 to 72 hours and lithium ionconductive glass-ceramics containing Li_(1+X) (Al, Ga)_(X) Ti_(2-X)(PO₄)₃ as a main crystal phase were thereby produced.

A heat treatment at a higher temperature within the above describedtemperature range will be desirable if micro cracks are not producedbecause a heat treatment at a higher temperature will reduce the heattreating time. Generally speaking, a heat treatment performed at atemperature which is higher by about 300° C. than a crystallizationtemperature of the glass will be most effective because it will providethe highest conductivity.

In the case of the glass-ceramics according to the invention made of theP₂ O₅ -SiO₂ -TiO₂ -(Al₂ O₃, Ga₂ O₃)-Li₂ O system, the above describedcomposition ranges have been selected because, within these compositionranges, dense glass-ceramics containing Li_(1+X+Y) M_(X) Ti_(2-X) Si_(Y)P_(3-Y) O₁₂ (where 0<X≦0.4, 0<Y≦0.6) as a main crystal phase andexhibiting a high lithium ion conductivity at room temperature wereobtained by heat treating glasses of the same composition ranges. It hasbeen found that the same crystal can be precipitated even in acomposition range outside of the above described composition ranges butthis crystal does not constitute a main crystal phase of a glass-ceramicproduced and conductivity of this glass-ceramic is not sufficientlyhigh.

In this system, SiO₂ is a very important ingredient. By adding SiO₂, theglass forming range is broadened and, moreover, melting property andthermal stability of the glass are improved and an excellentconductivity in the order of 10⁻³ S/cm can be obtained.

A part of Al or Ga may be replaced by one of such trivalent metalelements such as B, In, Sc, Fe and Cr or one of such divalent metalelements as Mg and Zn. Likewise, a part of Ti may be replaced by Zr anda part of Si may be replaced by Ge. In these cases, however, the amountof Al, Ga, Ti or Si replaced by such metal element should not exceed 5%.If the amount of the replaced metal element exceeds 5%, conductivitywill drop significantly.

For improving the melting property of the glass, As₂ O₃, Sb₂ O₃, Ta₂ O₅,CdO or PbO may be added. The amount of such ingredient however shouldnot exceed 3%. If the amount of such ingredient exceeds 3%, conductivityof the glass-ceramic will decrease as the amount of addition of theingredient increases.

A method for manufacturing the conductive glass-ceramics of the P₂ O₅-SiO₂ -TiO₂ -(Al₂ O₃, Ga₂ O₃)-Li₂ O system will now be described.

Starting materials are weighed at a predetermined ratio and mixeduniformly and the mixed materials are thereafter put in a platinumcrucible and heated and melted in an electric furnace. First, gascomponents coming from the raw materials are evaporated at 700° C. andthen the temperature is raised to 1400° C. to 1500° C. and the materialsare melted at this temperature for about one to two hours. Then the meltis cast onto stainless steel plate to form a sheet glass. The glass thusproduced is thereafter subjected to heat treatment by heating it under atemperature ranging from 680° C. to 800° C. for about twelve hours andsubsequently heating it under a temperature which is higher by 200° C.to 350° C. for about twenty-four hours and glass-ceramics containingLi_(1+X+Y) M_(X) Ti_(2-X) Si_(Y) P_(3-Y) O₁₂ as a main crystal phase andhaving a high lithium ion conductivity is produced.

This two-step heat treatment method is applicable also to the productionof the glass-ceramics of the P₂ O₅ -TiO₂ -(Al₂ O₃, Ga₂ O₃)-Li₂ O system.Conversely, the glass-ceramics of the P₂ O₅ -SiO₂ -TiO₂ -(Al₂ O₃, Ga₂O₃)-Li₂ O system can be produced by employing the one step heattreatment method described above with respect to the glass-ceramics ofthe P₂ O₅ -TiO₂ -(Al₂ O₃, Ga₂ O₃)-Li₂ O system.

Examples

Examples of the glass-ceramics made according to the invention will nowbe described. It should be noted that these examples are illustrativeonly and the scope of the invention in no way is restricted by theseexamples.

Example 1

As starting materials, NH₄ H₂ PO₄, TiO₂, Al(OH)₃ and Li₂ CO₃ were used.These starting materials were weighed to constitute a composition of39P₂ O₅ -8.5Al₂ O₃ -39TiO₂ -13.5Li₂ O in mol %. The materials were mixeduniformly and then put in a platinum crucible and heated and melted inan electric furnace. First, CO₂, NH₃ and H₂ O coming from the rawmaterials were evaporated at 700° C. Then the temperature was raised to1450° C. and the materials were melted by heating them at thistemperature for 1.5 hour. Thereafter, the melt was cast onto a stainlesssteel plate to form a uniform sheet glass. The glass was annealed at550° C. for two hours for removing thermal stress of the glass.

The glass thus produced was cut into specimens each having the size of20×20 mm. The specimens of glass were polished on both surfaces andsubjected to heat treatment under various heat conditions. The crystalphase which precipitated in the specimens was determined by the powderX-ray diffraction method. As a result, it was found that theprecipitated crystal phase under all heat conditions was Li_(1+X) Al_(X)Ti_(2-X) (PO₄)₃. Electrical conductivity of the glass-ceramic wasmeasured within a range from 10⁻² -3×10⁺⁷ Hz by the complex impedance.Resistance of the specimens (sum of grain resistance and grain boundaryresistance) was determined from the Cole-Cole Plot and the conductivitywas calculated by the equation σ=(t/A)(1/R) (where σ is conductivity, tis thickness of the specimen, A is electrode area and R is resistance ofthe specimen). As a result, the specimen which was heat treated at 1000°C. for 12 hours exhibited the highest conductivity of 1.3×10⁻³ S/Cm atroom temperature (Table 1, Example No. 1).

Example 2

As the starting materials, NH₄ H₂ PO₄, TiO₂, Al(OH)₃, Ga₂ O₃ and Li₂ CO₃were used to produce a glass-ceramic by employing the same manner as inExample 1. The crystal phase which grew in specimens of thisglass-ceramic was determined to be Li_(1+X) (Al, Ga)_(X) Ti_(2-X)(PO₄)₃. The specimen which was heat treated at 950° C. for 12 hoursexhibited the highest conductivity of 1.0×10⁻³ S/cm (Table 1, ExampleNo. 2).

                  TABLE 1                                                         ______________________________________                                        (composition in mol %)                                                                       Examples                                                       No.              1        2                                                   ______________________________________                                        P.sub.2 O.sub.5  39       39                                                  TiO.sub.2        39       38                                                  Al.sub.2 O.sub.3 8.5      6.5                                                 Ga.sub.2 O.sub.3          2.5                                                 Li.sub.2 O       13.5     14                                                  coductivity at room                                                                            1.3 × 10.sup.-3                                                                  1.0 × 10.sup.-3                               temperature (S/cm)                                                            temperature of   1000     950                                                 heat treatment (°C.)                                                   time of          12       12                                                  heat treatment (Hr)                                                           ______________________________________                                    

Example 3

As starting materials, NH₄ H₂ PO₄, SiO₂, TiO₂, Al(OH)₃ and Li₂ CO₃ wereused. These starting materials were weighed to constitute a compositionof 32P₂ O₅ -8SiO₂ -41TiO₂ -5Al₂ O₃ -14Li₂ O in mol %. The materials weremixed uniformly and then put in a platinum crucible and heated andmelted in an electrical furnace. First, CO₂, NH₃ and H₂ O coming fromthe raw materials were evaporated at 700° C. Then the temperature wasraised to 1450° C. and the materials were melted by heating them at thistemperature for 1.5 hour. Thereafter, the melt was cast onto a stainlesssteel plate to form a uniform sheet glass. The glass was annealed at550° C. for two hours for removing thermal stress of the glass.

The glass thus produced was cut into specimens each having the size of20×20 mm. The specimens of glass were polished on both surfaces andsubjected to heat treatment at a temperature of 800° C. for 12 hours andthen at 1000° C. for 24 hours to produce a dense glass-ceramic. Thecrystal phase precipitated in the specimens was determined by the powderX-ray diffraction to be Li_(1+X+Y) Al_(X) Ti_(2-X) Si_(Y) P_(3-Y) O₁₂.The glass-ceramic exhibited a very high conductivity of 1.5×10⁻³ S/cm atroom temperature (Table 2, Example No. 3).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an X-ray diffraction pattern of the glass-ceramic ofExample 3.

Examples 4-8

Specimens of glass-ceramics were prepared in a manner similar to Example3. The compositions and conductivities of these specimens as well as thecomposition and conductivity of Example 1 are shown in the followingTables 2 and 3.

                  TABLE 2                                                         ______________________________________                                        (composition in mol %)                                                                  Examples                                                            No.         3           4        5                                            ______________________________________                                        P.sub.2 O.sub.5                                                                           32          33.5     30                                           SiO.sub.2   8           6        10                                           TiO.sub.2   41          42       40                                           Al.sub.2 O.sub.3                                                                          5           5        5                                            Ga.sub.2 O.sub.3                                                              Li.sub.2 O  14          13.5     15                                           conductivity                                                                              1.5 × 10.sup.-3                                                                     1.0 × 10.sup.-3                                                                  1.2 × 10.sup.-3                        at room temper-                                                               ature (S/cm)                                                                  ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        (composition in mol %)                                                                  Examples                                                            No.         6           7        8                                            ______________________________________                                        P.sub.2 O.sub.5                                                                           35          32       35                                           SiO.sub.2   4           8        4                                            TiO.sub.2   38          41       38                                           Al.sub.2 O.sub.3                                                                          8                    5                                            Ga.sub.2 O.sub.3        5        3                                            Li.sub.2 O  15          14       15                                           conductivity                                                                              1.1 × 10.sup.-3                                                                     1.2 × 10.sup.-3                                                                  1.0 × 10.sup.-3                        at room temper-                                                               ature (S/cm)                                                                  ______________________________________                                    

What is claimed is:
 1. Lithium ion conductive glass-ceramics comprisingin mol %:

    ______________________________________                                        P.sub.2 O.sub.5     38-40%                                                    TiO.sub.2           25-45%                                                    M.sub.2 O.sub.3 (where M is Al or Ga)                                                              5-15%                                                    Li.sub.2 O          10-20%                                                    ______________________________________                                    

and containing Li_(1+X) (Al, Ga)_(X) Ti_(2-X) (PO₄)₃ where 0<X<0.8 as amain crystal phase.
 2. Glass-ceramics as defined in claim 1 wherein saidglass-ceramics comprise in mol %:

    ______________________________________                                        P.sub.2 O.sub.5    38-40%                                                     TiO.sub.2          35-45%                                                     M.sub.2 O.sub.3 (where M is Al or Ga)                                                             5-15%                                                     LiO.sub.2           10-16%.                                                   ______________________________________                                    


3. Glass-ceramics as defined in claim 1 wherein said glass-ceramicscomprise in mol %

    ______________________________________                                               P.sub.2 O.sub.5                                                                     38-40%                                                                  TiO.sub.2                                                                           25-45%                                                                  Ga.sub.2 O.sub.3                                                                     5-12%                                                                  Li.sub.2 O                                                                           10-20%.                                                         ______________________________________                                    


4. Lithium ion conductive glass-ceramics comprising in mol %

    ______________________________________                                        P.sub.2 O.sub.5    26-40%                                                     SiO.sub.2          0.5-12%                                                    TiO.sub.2          30-45%                                                     M.sub.2 O.sub.3 (where M is Al or Ga)                                                             5-10%                                                     Li.sub.2 O         10-18%                                                     ______________________________________                                    

and containing Li_(1+X+Y) M_(X) Ti_(2-X) Si_(Y) P_(3-Y) O₁₂ (where0<X≦0.4 and 0<Y≦0.6) as a main crystal phase.
 5. Glass-ceramics asdefined in claim 4 wherein said glass-ceramics comprise in mol %:

    ______________________________________                                        P.sub.2 O.sub.5    26-40%                                                     SiO.sub.2          0.5-12%                                                    TiO.sub.2          32-45%                                                     M.sub.2 O.sub.3 (where M is Al or Ga)                                                             5-10%                                                     LiO.sub.2           10-18%.                                                   ______________________________________                                    


6. Glass-ceramics as defined in claim 4 wherein said glass-ceramicscomprise in mol %

    ______________________________________                                               P.sub.2 O.sub.5                                                                     26-40%                                                                  SiO.sub.2                                                                           0.5-12%                                                                 TiO.sub.2                                                                           32-45%                                                                  Ga.sub.2 O.sub.3                                                                     5-10%                                                                  Li.sub.2 O                                                                           10-18%.                                                         ______________________________________                                    


7. Glass-ceramics as defined in claim 1 wherein M is Ga. 8.Glass-ceramics as defined in claim 2 wherein M is Ga.